Reconfigurable optical add-drop multiplexers employing polarization diversity

ABSTRACT

This invention provides a novel wavelength-separating-routing (WSR) apparatus that uses a diffraction grating to separate a multi-wavelength optical signal by wavelength into multiple spectral channels, which are focused onto an array of corresponding channel micromirrors. The channel micromirrors are individually controllable and continuously pivotable to reflect the spectral channels into selected output ports. As such, the inventive WSR apparatus is capable of routing the spectral channels on a channel-by-channel basis and coupling any spectral channel into any one of the output ports. The WSR apparatus of the invention may further employ a polarization diversity scheme, whereby polarization-sensitive effects become inconsequential and insertion loss is minimized. The WSR apparatus of the invention may additionally be equipped with servo-control and channel equalization capabilities. The WSR apparatus of the invention can be used to construct a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for WDM optical networking applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/938,426, filed on Aug. 23, 2001, now U.S. Pat. No. 6,625,346and which claims priority from U.S. Provisional Patent Application Ser.No. 60/277,217, filed on Mar. 19, 2001.

BACKGROUND

This invention relates generally to optical communication systems. Morespecifically, it relates to a novel class of dynamically reconfigurableoptical add-drop multiplexers (OADMs) for wavelength divisionmultiplexed optical networking applications.

As fiber-optic communication networks rapidly spread into every walk ofmodern life, there is a growing demand for optical components andsubsystems that enable the fiber-optic communications networks to beincreasingly scalable, versatile, robust, and cost-effective.

Contemporary fiber-optic communications networks commonly employwavelength division multiplexing (WDM), for it allows multipleinformation (or data) channels to be simultaneously transmitted on asingle optical fiber by using different wavelengths and therebysignificantly enhances the information bandwidth of the fiber. Theprevalence of WDM technology has made optical add-drop multiplexersindispensable building blocks of modern fiber-optic communicationnetworks. An optical add-drop multiplexer (OADM) serves to selectivelyremove (or drop) one or more wavelengths from a multiplicity ofwavelengths on an optical fiber, hence taking away one or more datachannels from the traffic stream on the fiber. It further adds one ormore wavelengths back onto the fiber, thereby inserting new datachannels in the same stream of traffic. As such, an OADM makes itpossible to launch and retrieve multiple data channels (eachcharacterized by a distinct wavelength) onto and from an optical fiberrespectively, without disrupting the overall traffic flow along thefiber. Indeed, careful placement of the OADMs can dramatically improvean optical communication network's flexibility and robustness, whileproviding significant cost advantages.

Conventional OADMs in the art typically employmultiplexers/demultiplexers (e.g., waveguide grating routers orarrayed-waveguide gratings), tunable filters, optical switches, andoptical circulators in a parallel or serial architecture to accomplishthe add and drop functions. In the parallel architecture, as exemplifiedin U.S. Pat. No. 5,974,207, a demultiplexer (e.g., a waveguide gratingrouter) first separates a multi-wavelength signal into its constituentspectral components. A wavelength switching/routing means (e.g., acombination of optical switches and optical circulators) then serves todrop selective wavelengths and add others. Finally, a multiplexercombines the remaining (i.e., the pass-through) wavelengths into anoutput multi-wavelength optical signal. In the serial architecture, asexemplified in U.S. Pat. No. 6,205,269, tunable filters (e.g., Braggfiber gratings) in combination with optical circulators are used toseparate the drop wavelengths from the pass-through wavelengths andsubsequently launch the add channels into the pass-through path. And ifmultiple wavelengths are to be added and dropped, additionalmultiplexers and demultiplexers are required to demultiplex the dropwavelengths and multiplex the add wavelengths, respectively.Irrespective of the underlying architecture, the OADMs currently in theart are characteristically high in cost, and prone to significantoptical loss accumulation. Moreover, the designs of these OADMs are suchthat it is inherently difficult to reconfigure them in a dynamicfashion.

U.S. Pat. No. 6,204,946 to Askyuk et al. discloses an OADM that makesuse of free-space optics in a parallel construction. In this case, amulti-wavelength optical signal emerging from an input port is incidentonto a ruled diffraction grating. The constituent spectral channels thusseparated are then focused by a focusing lens onto a linear array ofbinary micromachined mirrors. Each micromirror is configured to operatebetween two discrete states, such that it either retroreflects itscorresponding spectral channel back into the input port as apass-through channel, or directs its spectral channel to an output portas a drop channel. As such, the pass-through signal (i.e., the combinedpass-through channels) shares the same input port as the input signal.An optical circulator is therefore coupled to the input port, to providenecessary routing of these two signals. Likewise, the drop channelsshare the output port with the add channels. An additional opticalcirculator is thereby coupled to the output port, from which the dropchannels exit and the add channels are introduced into the output port.The add channels are subsequently combined with the pass-through signalby way of the diffraction grating and the binary micromirrors.

Although the aforementioned OADM disclosed by Askyuk et al. has theadvantage of performing wavelength separating and routing in free spaceand thereby incurring less optical loss, it suffers a number oflimitations. First, it requires that the pass-through signal share thesame port/fiber as the input signal. An optical circulator therefore hasto be implemented, to provide necessary routing of these two signals.Likewise, all the add and drop channels enter and leave the OADM throughthe same output port, hence the need for another optical circulator.Moreover, additional means must be provided to multiplex the addchannels before entering the system and to demultiplex the drop channelsafter exiting the system. This additional multiplexing/demultiplexingrequirement adds more cost and complexity that can restrict theversatility of the OADM thus-constructed. Second, the opticalcirculators implemented in this OADM for various routing purposesintroduce additional optical losses, which can accumulate to asubstantial amount. Third, the constituent optical components must be ina precise alignment, in order for the system to achieve its intendedpurpose. There are, however, no provisions provided for maintaining therequisite alignment; and no mechanisms implemented for overcomingdegradation in the alignment owing to environmental effects such asthermal and mechanical disturbances over the course of operation.

U.S. Pat. No. 5,906,133 to Tomlinson discloses an OADM that makes use ofa design similar to that of Aksyuk et al. There are input, output, dropand add ports implemented in this case. By positioning the four ports ina specific arrangement, each micromirror (being switchable between twodiscrete positions) either reflects its corresponding channel (comingfrom the input port) to the output port, or concomitantly reflects itschannel to the drop port and an incident add channel to the output port.As such, this OADM is able to perform both the add and drop functionswithout involving additional optical components (such as opticalcirculators used in the system of Aksyuk et al.). However, because asingle drop port is designated for all the drop channels and a singleadd port is designated for all the add channels, the add channels wouldhave to be multiplexed before entering the add port and the dropchannels likewise need to be demultiplexed upon exiting from the dropport. Moreover, as in the case of Askyuk et al., there are no provisionsprovided for maintaining requisite optical alignment in the system, andno mechanisms implemented for combating degradation in the alignment dueto environmental effects over the course of operation.

As such, the prevailing drawbacks suffered by the OADMs currently in theart are summarized as follows:

-   -   1) The wavelength routing is intrinsically static, rendering it        difficult to dynamically reconfigure these OADMs.    -   2) Add and/or drop channels often need to be multiplexed and/or        demultiplexed, thereby imposing additional complexity and cost.    -   3) Stringent fabrication tolerance and painstaking optical        alignment are required.

Moreover, the optical alignment is not actively maintained, rendering itsusceptible to environmental effects such as thermal and mechanicaldisturbances over the course of operation.

-   -   4) In an optical communication network, OADMs are typically in a        ring or cascaded configuration. In order to mitigate the        interference amongst OADMs, which often adversely affects the        overall performance of the network, it is essential that the        optical power levels of spectral channels entering and exiting        each OADM be managed in a systematic way, for instance, by        introducing power (or gain) equalization at each stage. Such a        power equalization capability is also needed for compensating        for non-uniform gain caused by optical amplifiers (e.g., erbium        doped fiber amplifiers) in the network. There lacks, however, a        systematic and dynamic management of the optical power levels of        various spectral channels in these OADMs.    -   5) The inherent high cost and optical loss further impede the        wide application of these OADMs.

In view of the foregoing, there is an urgent need in the art for opticaladd-drop multiplexers that overcome the aforementioned shortcomings in asimple, effective, and economical construction.

SUMMARY OF THE INVENTION

The invention provides a polarization diversitywavelength-separating-routing (WSR) apparatus and method which minimizesinsertion loss and polarization-dependent loss (PDL).

In WSR apparatus with which the invention may be used, amulti-wavelength optical signal is provided from an input port to awavelength-separator which separates the multi-wavelength optical signalby wavelength into multiple spectral channels. Each channel may becharacterized by a distinct center wavelength and associated bandwidth.A beam-focuser may focus the spectral channels into corresponding spotsonto a plurality of channel micromirrors positioned such that eachchannel micromirror receives one of the spectral channels. The channelmicromirrors are individually controllable and movable, e.g.,continuously pivotable or rotatable, so as to reflect the spectralchannels into selected ones of the output ports. Each output port mayreceive any number of the reflected spectral channels.

In one aspect, the WSR apparatus of the invention employs a polarizationdiversity arrangement to overcome polarization-sensitive effects theconstituent optical elements may possess. A polarization-displacing unitand a polarization-rotating unit may be disposed along the optical pathbetween the fiber collimators providing the input and output ports andthe wavelength-separator which separates the input multi-wavelengthoptical signal into the constituent wavelengths. Thepolarization-displacing unit decomposes the input multi-wavelengthoptical signal into first and second polarization components. Thepolarization-rotating unit may subsequently rotate the polarization ofthe second polarization component so that its polarization issubstantially parallel to the first polarization component, e.g., by90-degrees. The wavelength-separator separates the incident opticalsignals by wavelength into first and second sets of optical beams,respectively. The beam-focuser may focus the first and second sets ofoptical beams into corresponding focused spots, impinging onto thechannel micromirrors. The first and second optical beams associated withthe same wavelength may impinge onto (and be manipulated by) the samechannel micromirror. The channel micromirrors may be individuallycontrolled such that the first and second sets of optical beams aredeflected, upon reflection. The reflected first set of optical beams maysubsequently undergo a rotation in polarization by, e.g., 90 degrees, bythe polarization-rotating unit. This enables the polarization-displacingunit to recombine the reflected first and second sets of optical beamsby wavelength respectively into reflected spectral channels, prior tobeing coupled into the output ports.

The polarization-displacing unit may comprise one or morepolarization-displacing elements, each being a birefringent beamdisplacer, or a polarizing-beam-splitting element, e.g., a polarizingbeam splitter in conjunction with a suitable beam-reflector. Thepolarization-rotating unit may include one or more polarization rotatingelements, each being a half-wave plate, a Faraday rotator, or a liquidcrystal rotator known in the art.

A distinct feature of the channel micromirrors in the WSR apparatus isthat the motion of each channel micromirror is under analog control suchthat its pivoting angle can be continuously adjusted. This enables eachchannel micromirror to scan its corresponding spectral channel acrossall possible output ports and thereby direct the spectral channel to anydesired output port.

In the WSR apparatus, the wavelength-separator may be a ruleddiffraction grating, a holographic diffraction grating, an echellegrating, a curved diffraction grating, a transmission grating, adispersing prism, or other wavelength-separating means known in the art.The beam-focuser may be a single lens, an assembly of lenses, or otherbeam-focusing means known in the art. The channel micromirrors may besilicon micromachined mirrors, reflective ribbons (or membranes), orother types of beam-deflecting means known in the art. Each channelmicromirror may be pivotable about one or two axes. Fiber collimatorsserving as the input and output ports may be arranged in aone-dimensional or two-dimensional array. In the latter case, thechannel micromirrors may be pivotable biaxially.

In another aspect, the WSR apparatus of the invention may comprise anarray of collimator-alignment mirrors, in optical communication with thewavelength-separator and the fiber collimators, for adjusting thealignment of the input multi-wavelength signal and for directing thespectral channels into the selected output ports by way of angularcontrol of the collimated beams. Each collimator-alignment mirror may berotatable about one or two axes. The collimator-alignment mirrors may bearranged in a one-dimensional or two-dimensional array. First and secondarrays of imaging lenses may additionally be optically interposedbetween the collimator-alignment mirrors and the fiber collimators suchthat the collimator-alignment mirrors are effectively “imaged” onto thecorresponding fiber collimators to ensure an optimal alignment.

In another aspect, the WSR apparatus of the invention may include aservo-control assembly, in communication with the channel micromirrorsand the output ports. The servo-control assembly serves to monitor theoptical power levels of the spectral channels coupled into the outputports and further provide control of the channel micromirrors on anindividual basis, so as to maintain a predetermined coupling efficiencyof each spectral channel into one of the output ports. As such, theservo-control assembly provides dynamic control of the coupling of thespectral channels into the respective output ports and actively managesthe optical power levels of the spectral channels coupled into theoutput ports. (If the WSR apparatus includes an array ofcollimator-alignment mirrors as described above, the servo-controlassembly may additionally provide dynamic control of thecollimator-alignment mirrors.) Moreover, the utilization of such aservo-control assembly effectively relaxes the requisite fabricationtolerances and the precision of optical alignment during assembly of aSR apparatus of the invention, and further enables the system to correctfor shift in optical alignment over the course of operation. A WSRapparatus incorporating a servo-control assembly thus described istermed a WSR-S apparatus, in the following discussion.

The WSR apparatus of the invention affords a variety of optical devices,including a novel class of dynamically reconfigurable optical add-dropmultiplexers (OADMs), that provide many advantages over the prior artdevices, notably:

-   -   1) By advantageously employing an array of channel micromirrors        that are individually and continuously controllable, an OADM of        the invention is capable of routing the spectral channels on a        channel-by-channel basis and directing any spectral channel into        any one of the output ports. As such, its underlying operation        is dynamically reconfigurable, and its underlying architecture        is intrinsically scalable to a large number of channel counts.    -   2) The add and drop spectral channels need not be multiplexed        and demultiplexed before entering and after leaving the OADM        respectively. And there are not fundamental restrictions on the        wavelengths to be added or dropped.    -   3) The coupling of the spectral channels into the output ports        is dynamically controlled by a servo-control assembly, rendering        the OADM less susceptible to environmental effects (such as        thermal and mechanical disturbances) and therefore more robust        in performance. By maintaining an optimal optical alignment, the        optical losses incurred by the spectral channels are also        significantly reduced.    -   4) The optical power levels of the spectral channels coupled        into the output ports can be dynamically managed according to        demand, or maintained at desired values (e.g., equalized at a        predetermined value) by way of the servo-control assembly. This        spectral power-management capability as an integral part of the        OADM will be particularly desirable in WDM optical networking        applications.    -   5) The use of free-space optics provides a simple, low loss, and        cost-effective construction. Moreover, the utilization of the        servo-control assembly effectively relaxes the requisite        fabrication tolerances and the precision of optical alignment        during initial assembly, enabling the OADM to be simpler and        more adaptable in structure, and lower in cost and optical loss.    -   6) The use of a polarization diversity scheme renders the        polarization-sensitive effects inconsequential in the OADM. This        enables the OADM to minimize the insertion loss; and enhance        spectral resolution in a simple and cost-effective construction        (e.g., by making use of high-dispersion diffraction grating        commonly available in the art). The polarization diversity        scheme further allows the overall optical paths of the two        polarization components for each spectral channel to be        substantially equalized, thereby minimizing the        polarization-dependent loss. Such attributes would be        particularly desirable in WDM optical networking applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D show a first embodiment of a wavelength-separating-routing(WSR) apparatus with which the invention may be employed, and themodeling results demonstrating the performance of the WSR apparatus;

FIG. 2A depicts a second embodiment of a WSR apparatus with which theinvention may be employed;

FIGS. 2B-2C show a third embodiment of a WSR apparatus with which theinvention may be employed;

FIG. 3 shows a fourth embodiment of a WSR apparatus with which theinvention may be employed;

FIGS. 4A-4B show schematic illustrations of two embodiments of a WSR-Sapparatus comprising a WSR apparatus and a servo-control assembly,according to the invention;

FIG. 5 depicts an exemplary embodiment of an optical add-dropmultiplexer (OADM) according to the invention;

FIG. 6 shows an alternative embodiment of an OADM according to theinvention;

FIGS. 7A-7B depict a fifth embodiment of a WSR apparatus according tothe invention employing a polarization diversity arrangement;

FIGS. 7C-7D depict two exemplary embodiments of apolarization-displacing unit that may be used in the WSR apparatus shownin FIGS. 7A-7B;

FIG. 8A shows a sixth embodiment of a WSR apparatus according to theinvention, employing a polarization diversity arrangement;

FIG. 8B depicts a seventh embodiment of a WSR apparatus according to theinvention employing a polarization diversity arrangement; and

FIG. 8C shows an eighth embodiment of a WSR apparatus according to theinvention employing a polarization diversity arrangement.

DETAILED DESCRIPTION

In this specification and appended claims, a “spectral channel” ischaracterized by a distinct center wavelength and associated bandwidth.Each spectral channel may carry a unique information signal, as in WDMoptical networking applications.

FIG. 1A depicts a first embodiment of a wavelength-separating-routing(WSR) apparatus with which the invention may be employed. By way ofexample to illustrate the general principles and the topologicalstructure of a wavelength-separating-routing (WSR) apparatus of theinvention, the WSR apparatus 100 comprises multiple input/output portswhich may be in the form of an array of fiber collimators 110, providingan input port 110-1 and a plurality of output ports 110-2 through 110-N(N≧3); a wavelength-separator which in one form may be a diffractiongrating 101; a beam-focuser in the form of a focusing lens 102; and anarray of channel micromirrors 103.

In operation, a multi-wavelength optical signal emerges from the inputport collimator 110-1. The diffraction grating 101 angularly separatesthe multi-wavelength optical signal into multiple spectral channels,which are in turn focused by the focusing lens 102 into a spatial arrayof distinct spectral spots (not shown in FIG. 1A) in a one-to-onecorrespondence. The channel micromirrors 103 are positioned inaccordance with the spatial array formed by the spectral spots, suchthat each channel micromirror receives one of the spectral channels. Thechannel micromirrors 103 are individually controllable and movable,e.g., pivotable (or rotatable) under analog (or continuous) control,such that, upon reflection, the spectral channels are directed intoselected ones of the output ports 110-2 through 110-N by way of thefocusing lens 102 and the diffraction grating 101. As such, each channelmicromirror is assigned to a specific spectral channel, hence the name“channel micromirror”. Each output port may receive any number of thereflected spectral channels.

For purposes of illustration and clarity, only a select few, e.g.,three, of the spectral channels, along with the input multi-wavelengthoptical signal, are graphically illustrated in FIG. 1A and the followingfigures. It should be noted, however, that there can be any number ofthe spectral channels in a WSR apparatus of the invention (so long asthe number of spectral channels does not exceed the number of channelmirrors employed in the system). It should also be noted that theoptical beams representing the spectral channels shown in FIG. 1A andthe following figures are provided for illustrative purpose only. Thatis, their sizes and shapes may not be drawn according to scale. Forinstance, the input beam and the corresponding diffracted beamsgenerally have different cross-sectional shapes, so long as the angle ofincidence upon the diffraction grating is not equal to the angle ofdiffraction, as is known to those skilled in the art.

In the embodiment of FIG. 1A, it is preferable that the diffractiongrating 101 and the channel micromirrors 103 are placed respectively atthe first and second, i.e., the front and back, focal planes (on theopposing sides) of the focusing lens 102. Such a telecentric arrangementallows the chief rays of the focused beams to be parallel to each otherand generally parallel to the optical axis. This telecentricconfiguration further allows the reflected spectral channels to beefficiently coupled into the respective output ports, thereby minimizingvarious translational walk-off effects that may otherwise arise.Moreover, the input multi-wavelength optical signal is preferablycollimated and circular in cross-section.

The corresponding spectral channels diffracted from the diffractiongrating 101 are generally elliptical in cross-section; they may be ofthe same size as the input beam in one dimension and elongated in theother dimension.

It is known that the diffraction efficiency of a diffraction grating isgenerally polarization-dependent. That is, the diffraction efficiency ofa grating in a standard mounting configuration may be considerablyhigher for P-polarization that is perpendicular to the groove lines onthe grating than for S-polarization that is orthogonal toP-polarization, especially as the number of groove lines (per unitlength) increases. To mitigate such polarization-sensitive effects, aquarter-wave plate 104 may be optically interposed between thediffraction grating 101 and the channel micromirrors 103, and preferablyplaced between the diffraction grating 101 and the focusing lens 102 asis shown in FIG. 1A. In this way, each spectral channel experiences atotal of approximately 90-degree rotation in polarization upontraversing the quarter-wave plate 104 twice. (That is, if a beam oflight has P-polarization when first encountering the diffractiongrating, it would have predominantly (if not all) S-polarization uponthe second encountering, and vice versa.) This ensures that all thespectral channels incur nearly the same amount of round-trippolarization dependent loss.

In the WSR apparatus 100 of FIG. 1A, the diffraction grating 101, by wayof example, is oriented such that the focused spots of the spectralchannels fall onto the channel micromirrors 103 in a horizontal array,as illustrated in FIG. 1B.

FIG. 1B is a close-up view of the channel micromirrors 103 shown in theembodiment of FIG. 1A. By way of example, the channel micromirrors 103are arranged in a one-dimensional array along the x-axis (i.e., thehorizontal direction in the figure), so as to receive the focused spotsof the spatially separated spectral channels in a one-to-onecorrespondence. (As in the case of FIG. 1A, only three spectral channelsare illustrated, each represented by a converging beam.) The reflectivesurface of each channel micromirror lies in the x-y plane as defined inthe figure and is movable, e.g., pivotable or deflectable about an axisalong the x-direction in an analog, i.e., continuous, manner. Eachspectral channel, upon reflection, is deflected in the y-direction,e.g., downward, relative to its incident direction, so as to be directedinto one of the output ports 110-2 through 110-N shown in FIG. 1A.

As described above, a unique feature of the invention is that the motionof each channel micromirror is individually and continuouslycontrollable, such that its position, e.g., pivoting angle, can becontinuously adjusted. This enables each channel micromirror to scan itscorresponding spectral channel across all possible output ports andthereby direct the spectral channel to any desired output port. Toillustrate this capability, FIG. 1C shows a plot of coupling efficiencyas a function of a channel micromirror's pivoting angle θ, provided by aray-tracing model of a WSR apparatus in the embodiment of FIG. 1A. Asused herein, the coupling efficiency for a spectral channel is definedas the ratio of the amount of optical power coupled into the fiber corein an output port to the total amount of optical power incident upon theentrance surface of the fiber (associated with the fiber collimatorserving as the output port). In the ray-tracing model, the input opticalsignal is incident upon a diffraction grating with 700 lines permillimeter at a grazing angle of 85 degrees, where the grating is blazedto optimize the diffraction efficiency for the “−1” order. The focusinglens has a focal length of 100 mm. Each output port is provided by aquarter-pitch GRIN lens, e.g., 2 mm in diameter, coupled to an opticalfiber (see FIG. 1D). As displayed in FIG. 1C, the coupling efficiencyvaries with the pivoting angle θ, and it requires about a 0.2-degreechange in θ for the coupling efficiency to become practically negligiblein this exemplary case. As such, each spectral channel may practicallyacquire any coupling efficiency value by way of controlling the pivotingangle of its corresponding channel micromirror. This is also to say thatvariable optical attenuation at the granularity of a single wavelengthcan be obtained in a WSR apparatus of the invention. FIG. 1D providesray-tracing illustrations of two extreme points on the couplingefficiency vs. θ curve of FIG. 1C: on-axis coupling corresponding toθ=0, where the coupling efficiency is maximum; and off-axis couplingcorresponding to θ=0.2 degrees, where the representative collimated beam(representing an exemplary spectral channel) undergoes a significanttranslational walk-off and renders the coupling efficiency practicallynegligible. The exemplary modeling results thus described demonstratethe unique capabilities of the WSR apparatus of the invention.

FIG. 1A is but one of many embodiments of a WSR apparatus with which theinvention may be used. In general, the wavelength-separator is awavelength-separating means that may be a ruled diffraction grating, aholographic diffraction grating, an echelle grating, a dispersing prism,or other types of spectral-separating means known in the art. Thebeam-focuser may be a focusing lens, an assembly of lenses, or otherbeam-focusing means known in the art. The focusing function may also beaccomplished by using a curved diffraction grating as thewavelength-separator. The channel micromirrors may be provided bysilicon micromachined mirrors, reflective ribbons (or membranes), orother types of beam-deflecting elements known in the art. Eachmicromirror may be pivoted about one or two axes. It is important thatthe pivoting (or rotational) motion of each channel micromirror beindividually controllable in an analog manner, whereby the pivotingangle can be continuously adjusted so as to enable the channelmicromirror to scan a spectral channel across all possible output ports.The underlying fabrication techniques for micromachined mirrors andassociated actuation mechanisms are well documented in the art, see U.S.Pat. No. 5,629,790 for example. Moreover, a fiber collimator istypically in the form of a collimating lens (such as a GRIN lens) and aferrule-mounted fiber packaged together in a mechanically rigid(stainless steel or glass) tube. The fiber collimators serving as theinput and output ports may be arranged in a one-dimensional array, atwo-dimensional array, or other desired spatial pattern. For instance,they may be conveniently mounted in a linear array along a V-groovefabricated on a substrate made of silicon, plastic, or ceramic, ascommonly practiced in the art. It should be noted, however, that theinput port and the output ports need not necessarily be in close spatialproximity with each other, such as in an array configuration, althoughclose packing would reduce the rotational range required for eachchannel micromirror. Those skilled in the art will know how to design aWSR apparatus according to the invention, to best suit a givenapplication.

A WSR apparatus embodying the invention may further comprise an array ofcollimator-alignment mirrors, for adjusting the alignment of the inputmulti-wavelength optical signal and facilitating the coupling of thespectral channels into the respective output ports, as shown in FIGS.2A-2B and 3.

FIG. 2A depicts a second embodiment of a WSR with which the inventionmay be used. By way of example, WSR apparatus 200 is built upon andhence shares a number of the elements used in the embodiment of FIG. 1A,as identified by those elements labeled with identical numerals.Moreover, a one-dimensional array 220 of collimator-alignment mirrors220-1 through 220-N is optically interposed between the diffractiongrating 101 and the fiber collimator array 110. The collimator-alignmentmirror 220-1 is designated to correspond with the input port 110-1, foradjusting the alignment of the input multi-wavelength optical signal andtherefore ensuring that the spectral channels impinge onto thecorresponding channel micromirrors. The collimator-alignment mirrors220-2 through 22-N are designated to the output ports 110-2 through110-N in a one-to-one correspondence, serving to provide angular controlof the collimated beams of the reflected spectral channels and therebyfacilitating the coupling of the spectral channels into the respectiveoutput ports according to desired coupling efficiencies. Eachcollimator-alignment mirror may be rotatable about one axis, or twoaxes.

The embodiment of FIG. 2A is attractive in applications where the fibercollimators (serving as the input and output ports) are desired to beplaced in close proximity to the collimator-alignment mirror array 220.To best facilitate the coupling of the spectral channels into the outputports, arrays of imaging lenses may be implemented between thecollimator-alignment mirror array 220 and the fiber collimator array110, as depicted in FIG. 2B. By way of example, WSR apparatus 250 ofFIG. 2B is built upon and hence shares many of the elements used in theembodiment of FIG. 2A, as identified by those elements labeled withidentical numerals. Additionally, first and second arrays 260, 270 ofimaging lenses are placed in a 4-f telecentric arrangement with respectto the collimator-alignment mirror array 220 and the fiber collimatorarray 110. The dashed box 280 shown in FIG. 2C provides a top view ofsuch a telecentric arrangement. In this case, the imaging lenses in thefirst and second arrays 260, 270 all have-the same focal length f. Thecollimator-alignment mirrors 220-1 through 220-N are placed at therespective first (or front) focal points of the imaging lenses in thefirst array 260. Likewise, the fiber collimators 110-1 through 110-N areplaced at the respective second (or back) focal points of the imaginglenses in the second array 270. The separation between the first andsecond arrays 260, 270 of imaging lenses is 2f. In this way, thecollimator-alignment mirrors 220-1 through 220-N are effectively imagedonto the respective entrance surfaces (i.e., the front focal planes) ofthe GRIN lenses in the corresponding fiber collimators 110-1 through110-N. Such a 4-f relay (or imaging) system substantially eliminatestranslational walk-off of the collimated beams at the output ports thatmay otherwise occur as the mirror angles change.

FIG. 3 shows a fourth embodiment of a WSR apparatus with which theinvention may be used. By way of example, WSR apparatus 300 is builtupon and hence shares a number of the elements used in the embodiment ofFIG. 2B, as identified by those elements labeled with identicalnumerals. In this case, the one-dimensional fiber collimator array 110of FIG. 2B is replaced by a two-dimensional array 350 of fibercollimators, providing for an input-port and a plurality of outputports. Accordingly, the one-dimensional collimator-alignment mirrorarray 220 of FIG. 2B is replaced by a two-dimensional array 320 ofcollimator-alignment mirrors, and first and second one-dimensionalarrays 260, 270 of imaging lenses of FIG. 2B are likewise replaced byfirst and second two-dimensional arrays 360, 370 of imaging lensesrespectively. As in the case of the embodiment of FIG. 2B, the first andsecond two-dimensional arrays 360, 370 of imaging lenses are placed in a4-f relay (or imaging) arrangement with respect to the two-dimensionalcollimator-alignment mirror array 320 and the two-dimensional fibercollimator array 350. Each of the channel micromirrors 103 must bepivotable biaxially in this case (in order to direct its correspondingspectral channel to any one of the output ports). As such, the WSRapparatus 300 is equipped to support a greater number of the outputports.

In addition to facilitating the coupling of the spectral channels intothe respective output ports as described above, the collimator-alignmentmirrors in the above embodiments also serve to compensate formisalignment, e.g., due to fabrication and assembly errors, in the fibercollimators that provide for the input and output ports. For instance,relative misalignment between the fiber cores and their respectivecollimating lenses in the fiber collimators can lead to pointing errorsin the collimated beams, which may be corrected for by thecollimator-alignment mirrors. For these reasons, thecollimator-alignment mirrors are preferably rotatable about two axes.They may be silicon micromachined mirrors, for fast rotational speeds.They may also be other types of mirrors or beam-deflecting elementsknown in the art.

To optimize the coupling of the spectral channels into the output portsand further maintain the optimal optical alignment against environmentaleffects such as temperature variations and mechanical instabilities overthe course of operation, a WSR apparatus of the invention mayincorporate a servo-control assembly, for providing dynamic control ofthe coupling of the spectral channels into the respective output portson a channel-by-channel basis. A WSR apparatus incorporating aservo-control assembly is termed a WSR-S apparatus within thisspecification.

FIG. 4A depicts a schematic illustration of a first embodiment of aWSR-S apparatus according to the invention. The WSR-S apparatus 400comprises a WSR apparatus 410 and a servo-control assembly 440. The WSR410 may be substantially identical to the apparatus 100 of FIG. 1A, orany other embodiment in accordance with the invention. The servo-controlassembly 440 includes a spectral monitor 460, for monitoring the opticalpower levels of the spectral channels coupled into the output ports420-1 through 420-N of the WSR apparatus 410. By way of example, thespectral monitor 460 is coupled to the output ports 420-1 through 420-Nby way of fiber-optic couplers 420-1-C through 420-N-C, wherein eachfiber-optic coupler serves to “tap off” a predetermined fraction of theoptical signal in the corresponding output port. The servo-controlassembly 440 further includes a processing unit 470, in communicationwith the spectral monitor 460 and the channel micromirrors 430 of theWSR apparatus 410. The processing unit 470 uses the optical powermeasurements from the spectral monitor 460 to provide feedback controlof the channel micromirrors 430 on an individual basis, so as tomaintain a desired coupling efficiency for each spectral channel into aselected output port. As such, the servo-control assembly 440 providesdynamic control of the coupling of the spectral channels into therespective output ports on a channel-by-channel basis and therebymanages the optical power levels of the spectral channels coupled intothe output ports. The optical power levels of the spectral channels inthe output ports may be dynamically managed according to demand, ormaintained at desired values, e.g., equalized at a predetermined value,in the invention. Such a spectral power-management capability isessential in WDM optical networking applications, as discussed above.

FIG. 4B depicts a schematic illustration of a second embodiment of aWSR-S apparatus according to the invention. The WSR-S apparatus 450comprises a WSR apparatus 480 and a servo-control assembly 490. Inaddition to the channel micromirrors 430 (and other elements identifiedby the same numerals as those used in FIG. 4A), the WSR apparatus 480further includes a plurality of collimator-alignment mirrors 485, andmay be configured according to the embodiment of FIGS. 2A, 2B, 3, or anyother embodiment in accordance with the invention. By way of example,the servo-control assembly 490 includes the spectral monitor 460 asdescribed in the embodiment of FIG. 4A, and a processing unit 495. Inthis case, the processing unit 495 is in communication with the channelmicromirrors 430 and the collimator-alignment mirrors 485 of the WSRapparatus 480, as well as the spectral monitor 460. The processing unit495 uses the optical power measurements from the spectral monitor 460 toprovide dynamic control of the channel micromirrors 430 along with thecollimator-alignment mirrors 485, so as to maintain the couplingefficiencies of the spectral channels into the output ports at desiredvalues.

In the embodiment of FIG. 4A or 4B, the spectral monitor 460 may be anyone of the spectral power monitoring devices known in the art that iscapable of detecting the optical power levels of spectral components ina multi-wavelength optical signal. Such devices are typically in theform of a wavelength-separating means, e.g., a diffraction grating, thatspatially separates a multi-wavelength optical signal by wavelength intoconstituent spectral components, and one or more optical sensors, e.g.,an array of photodiodes, that are configured such to detect the opticalpower levels of these spectral components. The processing unit 470 inFIG. 4A (or the processing unit 495 in FIG. 4B) typically includeselectrical circuits and signal processing programs for processing theoptical power measurements received from the spectral monitor 460 andgenerating appropriate control signals to be applied to the channelmicromirrors 430 (and the collimator-alignment mirrors 485 in the caseof FIG. 4B), so as to maintain the coupling efficiencies of the spectralchannels into the output ports at desired values. The electroniccircuitry and the associated signal processing algorithm/software for aprocessing unit in a servo-control system are known in the art. Askilled artisan would know how to implement a suitable spectral monitoralong with an appropriate processing unit to provide a servo-controlassembly in a WSP-S apparatus according to the invention, for a givenapplication.

The incorporation of a servo-control assembly provides additionaladvantages of effectively relaxing the requisite fabrication tolerancesand the precision of optical alignment during initial assembly of a WSRapparatus of the invention, and further enabling the system to correctfor shift in the alignment over the course of operation. By maintainingan optimal optical alignment, the optical losses incurred by thespectral channels are also significantly reduced. As such, the WSR-Sapparatus thus constructed is simpler and more adaptable in structure,more robust in performance, and lower in cost and optical loss.Accordingly, the WSR-S (or WSR) apparatus of the invention may be usedto construct a variety of optical devices and utilized in manyapplications. Moreover, a novel class of optical add-drop multiplexers(OADMs) may be built upon the WSR-S (or WSR) apparatus of the invention,as exemplified in the following embodiments.

FIG. 5 depicts an exemplary embodiment of an optical add-dropmultiplexer (OADM) according to the invention. By way of example, OADM500 comprises a WSR-S (or WSR) apparatus 510 and an optical combiner550. An input port 520 of the WSR-S apparatus 510 receives amulti-wavelength optical signal. The constituent spectral channels ofthis optical signal are subsequently separated and routed into aplurality of output ports, including a pass-through port 530 and one ormore drop ports 540-1 through 540-N (N≧1). The pass-through port 530 mayreceive any number of the spectral channels, i.e., the pass-throughspectral channels. Each drop port may also receive any number of thespectral channels, i.e., the drop spectral channels. The pass-throughport 530 is optically coupled to the optical combiner 550, which servesto combine the pass-through spectral channels with one or more addspectral channels provided by one or more add ports 560-1 through 560-M(M≧1). The combined optical signal is then routed into an existing port570, providing an output multi-wavelength optical signal.

In the above embodiment, the optical combiner 550 may be a K×1 (K≧2)broadband fiber-optic coupler, wherein there are K input-ends and oneoutput-end. The pass-through spectral channels and the add spectralchannels are fed into the K input-ends, e.g., in a one-to-onecorrespondence, and the combined optical signal exits from theoutput-end of the K×1 fiber-optic coupler as the output multi-wavelengthoptical signal of the system. Such a multiple-input coupler also servesthe purpose of multiplexing a multiplicity of add spectral channels tobe coupled into the OADM 500. If the optical power levels of thespectral channels in the output multi-wavelength optical signal aredesired to be actively managed, such as being equalized at apredetermined value, two spectral monitors may be utilized. As a way ofexample, the first spectral monitor may receive optical signals tappedoff from the pass-through port 530 and the drop ports 540-1 through540-N, e.g., by way of fiber-optic couplers as depicted in FIG. 4A or4B. The second spectral monitor receives optical signals tapped off fromthe exiting port 570. A servo-control system may be constructedaccordingly for monitoring and controlling the pass-through, drop andadd spectral channels. As such, the embodiment of FIG. 5 provides aversatile optical add-drop multiplexer in a simple and low-costassembly, while providing multiple physically separate drop/add ports ina dynamically reconfigurable fashion.

FIG. 6 depicts an alternative embodiment of an optical add-dropmultiplexer (OADM) according to the invention. By way of example, OADM600 comprises a first WSR-S apparatus 610 optically coupled to a secondWSR-S apparatus 650. Each WSR-S apparatus may be substantially identicalto the embodiment of FIG. 4A or 4B. (A WSR apparatus of the embodimentof FIG. 1A, 2A, 2B, or 3 may be alternatively implemented.) The firstWSR-S apparatus 610 includes an input port 620, a pass-through port 630,and one or more drop ports 640-1 through 640-N (N≧1). The pass-throughspectral channels from the pass-through port 630 are further coupled tothe second WSR-S apparatus 650, along with one or more add spectralchannels emerging from add ports 660-1 through 660-M (M≧1). In thisexemplary case, the pass-through port 630 and the add ports 660-1through 660-M constitute the input ports for the second WSR-S apparatus650. By way of its constituent wavelength-separator, e.g., a diffractiongrating, and channel micromirrors (not shown in FIG. 6), the secondWSR-S apparatus 650 serves to multiplex the pass-through spectralchannels and the add spectral channels, and route the multiplexedoptical signal into an exiting port 670 to provide an output signal ofthe system.

In the embodiment of FIG. 6, one WSR-S apparatus, e.g., the first WSR-Sapparatus 610, effectively performs dynamic drop function, whereas theother WSR-S apparatus (e.g., the second WSR-S apparatus 650) carries outdynamic add function. And there are essentially no fundamentalrestrictions on the wavelengths that can be added or dropped (other thanthose imposed by the overall communication system). Moreover, theunderlying OADM architecture thus presented is intrinsically scalableand can be readily extended to any number of cascaded WSR-S (or WSR)systems, if so desired for performing intricate add and drop functions.Additionally, the OADM of FIG. 6 may be operated in reverse direction,by using the input ports as the output ports, the drop ports as the addports, and vice versa.

As discussed above, the diffraction efficiency of a diffraction gratingis polarization-sensitive, and such polarization-sensitive effects maygive rise to significant insertion loss and polarization-dependent loss(PDL) in an optical system. The situation is further exacerbated in WDMoptical networking applications, where the polarization state of WDMsignals is typically indeterminate and may vary with time. This canproduce an undesirable time-varying insertion loss that may cause theoptical signals to fall below acceptable levels or render them unusable.Thus, it is desirable to avoid such polarization-sensitive effects, andthe invention affords a polarization diversity scheme that addressesthis, as will now be described.

FIG. 7A depicts a schematic top view and FIG. 7B depicts a schematicside view of a fifth embodiment of a WSR apparatus of the invention thatemploys a polarization diversity arrangement that minimizespolarization-sensitive effects. (The schematic top and side views inFIGS. 7A-7B and the following figures are presented with respect to theperspective view of FIG. 1A.) WSR apparatus 700 may make use of thegeneral architecture and a number of the elements used in the embodimentof FIG. 1A, as indicated by those elements labeled with the samenumerals. The input port 110-1 provides a multi-wavelength opticalsignal, which may be of indeterminate time varying polarization andwhich may contain wavelengths λ₁ through λ_(M), for instance, to apolarization-displacing unit 720. The polarization-displacing unit maybe disposed along the optical path between the array of fibercollimators 110 (including the input port 110-1 and the output ports110-2 through 110-N, as shown in FIG. 7B below) and the diffractiongrating 101. The polarization-displacing unit 720 serves to separate ordecompose the input multi-wavelength optical signal into a firstp-polarization component and a second orthogonal s-polarizationcomponent. Assuming that p-polarization is the “preferred” polarizationdirection of the diffraction grating 101, i.e., the diffractionefficiency is higher for the p-polarization component than for thes-polarization component, the p-polarization component of the inputoptical signal maybe output as a first optical signal 722 from thepolarization displacing unit. The second s-polarization component of theinput optical signal may be rotated by 90-degrees, by apolarization-rotating unit 730 to produce a second optical signal 732also having p-polarization. Thus, the two optical signals 722, 732incident onto the diffraction grating 101 both possess p-polarization.

The first and second polarization components (optical signals 722, 732)emerging from the polarization-displacing unit 720 and thepolarization-rotating unit 730, respectively, may undergo an unamorphicbeam magnification by a beam-modifying unit 740 and emerge as spatiallyseparated and magnified beams 742, 744 which impinge upon thediffraction grating 101. The configuration may be such that thebeam-modifying unit 740 preferentially enlarges the beam size in thedirection perpendicular to the groove lines on the diffraction grating101. This magnifies the optical beams in a direction perpendicular tothe groove lines of the grating so that the focused beams produced bythe focusing lens 102 are narrower in this direction, i.e.,perpendicular to the groove lines. This enables use, for example, ofrectangular shaped micromirrors. The diffraction grating 101subsequently separates the magnified first and second polarizationcomponents 742, 744 by wavelength into first and second sets ofdiffracted optical beams. Each set of optical beams comprises multiplewavelengths λ₁ through λ_(M), which are diffracted by the diffractiongrating 101 at different angles. The focusing lens 102 in turn focusesthe diffracted optical beams into corresponding focused spots whichimpinge onto the channel micromirrors 103. Each focused spot may beelliptical in cross-section. Further, the first and second diffractedoptical beams having the same wavelength, e.g., λ_(i), are arranged toimpinge onto the same channel micromirror, e.g., the channel micromirror103-i, see FIG. 7B. In this way, each channel micromirror handlesconcurrently two optical beams having the same polarization andwavelength.

FIG. 7B depicts a schematic side view of the WSR apparatus 700, whereonly the second multiple wavelength polarization component 732, 744 (onthe forward path), along with the reflected first set of optical beams(on the return path), are shown. For purposes of illustration andclarity, several channel micromirrors are explicitly identified in thisfigure, while the array of channel micromirrors as a whole is alsoindicated by the numeral 103. As described above with respect to FIGS.1A-1B, the channel micromirrors 103 are individually controllable andmovable, e.g., pivotable about an axis 750 (which may be parallel to thex-axis shown in FIG. 1B and perpendicular to the plane of FIG. 7B).Hence, each channel micromirror is capable of directing itscorresponding optical beams into any one of the output ports 110-2through 110-N byway of its pivoting motion. By way of example, thechannel micromirror 103-k may be controlled to direct the first andsecond optical beams with wavelength λ_(k) into the first output port110-2; the channel micromirror 103-j may be controlled to direct thefirst and second optical beams with wavelength λ_(j) into the secondoutput port 110-3; the channel micromirror 103-i may be controlled todirect the first and second optical beams with wavelength λ_(i) into thethird output port 110-4, and so on. Note that a plurality of the channelmicromirrors may be individually controlled to direct theircorresponding reflected optical beams into the same output port.

Referring to FIG. 7A, the first and second sets of optical beamsreflected from the respective channel micromirrors 103 are deflected outof the plane of the figure (as indicated by the side view of FIG. 7B);hence they are not explicitly shown in the top view of FIG. 7A. Withreference to FIG. 7B, it will be apparent to those skilled in the artthat the reflected first and second sets of optical beams each undergoan anamorphic beam demagnification by way of the beam-modifying unit740, thereby resuming the beam size of the input optical signal. Thereflected first set of optical beams subsequently undergoes a 90-degreepolarization rotation by the polarization-rotating unit 730, whereby thereflected first and second sets of optical beams are polarized in twoorthogonal directions upon entering the polarization-displacing unit720. This enables the polarization-displacing unit 720 to recombine thereflected first and second sets of optical beams by wavelengthrespectively into reflected spectral channels, prior to being coupledinto selected ones of the output ports 110-2 through 110-N.

It should be appreciated that the rotation in polarization produced by apolarization-rotating element, e.g., the polarization-rotating unit 730,may have slight variations about a prescribed angle, e.g., 90-degrees,due to imperfections that may exist in a practical system. Suchvariations, however, will not significantly affect the overallperformance of the invention.

In the embodiment of FIGS. 7A-7B, the polarization-displacing unit 720may be in the form of a single polarization-displacing element,corresponding to the array of fiber collimators 110. FIG. 7C shows twoschematic views of an exemplary embodiment of a polarization-displacingelement 720A which may be a birefringent beam displacer well known inthe art. The first schematic represented by dashed box 761 of FIG. 7Cillustrates a top view of the polarization-displacing element 720A,where an incident optical beam 770, e.g., the multi-wavelength opticalsignal in the embodiment of FIGS. 7A-7B, is decomposed into first andsecond polarization components 772, 774 polarized in two orthogonaldirections, as illustrated in the figure. Notice that the twopolarization components are spatially displaced and propagate inparallel, upon emerging from the polarization-displacing element 720A.The second schematic represented by dashed box 762 of FIG. 7C depicts anexemplary cross-sectional top view of the polarization-displacingelement 720A, where two parallel optical beams 776, 778 polarized in twoorthogonal directions, e.g., the first and second optical beamsassociated with wavelength λ_(i) in the embodiment of FIGS. 7A-7B, arerecombined by way of traversing the polarization-displacing element 720Ainto a single optical beam 780, e.g., the reflected spectral channelwith wavelength λ_(i) in the embodiment of FIGS. 7A-7B. As such, thepolarization-displacing element 720A acts as a polarization-separatingelement for optical beams propagating in one direction; and serves as apolarization-combining element for optical beams traversing in theopposite direction.

Those skilled in the art will appreciate that rather than using abirefringent beam displacer, the polarization-displacing element 720Amay alternatively be provided by a suitable polarizing-beam-splittingelement, e.g., a polarizing beam splitter commonly used in the art alongwith an appropriate beam-deflector or prism (such that the two emergingpolarization components propagate in parallel). Such apolarizing-beam-splitting element provides a substantially similarfunction to the aforementioned birefringent beam displacer. In general,a polarization-displacing element in the invention may be embodied byany optical element that provides a dual function of polarizationseparating and combining, as depicted in FIG. 7C.

Likewise, the polarization-rotating unit 730 may comprise a singlepolarization-rotating element, e.g., a half-wave plate, a liquid crystalrotator, a Faraday rotator, or any other means known in the art that iscapable of rotating the polarization of an optical beam by a prescribedangle, e.g., 90 degrees.

Alternatively, the polarization-displacing unit 720 may comprise aplurality of polarization-displacing elements, each corresponding to oneor more fiber collimators 110 in the embodiment of FIGS. 7A-7B. By wayof example, FIG. 7D depicts a schematic side view of apolarization-displacing unit 720B which may be an array ofpolarization-displacing elements 720-1 through 720-N. Eachpolarization-displacing element may be a birefringent beam displacer, apolarizing-beam-splitting element, or any other suitable means known inthe art, as described above with respect to FIG. 7C. In this case, thepolarization-rotating unit 730 may include one or morepolarization-rotating elements, each as described above. As a way ofexample, FIG. 7D also shows a schematic side view of apolarization-rotating unit 730B as an array of polarization-rotatingelements 730-1 through 730-N, which may be in a one-to-onecorrespondence with the polarization-displacing elements 720-1 through720-N. As such, the polarization-displacing unit 720B, along with thepolarization-rotating unit 730B, may be implemented in the embodiment ofFIGS. 7A-7B so that the polarization-displacing elements 720-1 through720-N are in a one-to-one correspondence with the fiber collimators 100that provide the input port 110-1 and the output ports 110-2 through110-N.

Those skilled in the art will appreciate that the exemplary embodimentsof FIGS. 7C-7D are provided as an example to illustrate how apolarization-displacing unit, along with a polarization-rotating unit,may be configured and operated in the invention. Various changes andmodifications may be made in this embodiment to perform the designatedfunctions in a substantially equivalent manner. For example, thepolarization-displacing unit 720, along with the polarization-rotatingunit 730, may alternatively be configured such that the first and secondpolarization components are spatially separated along a verticaldirection that is substantially perpendicular to the plane of the paperin the schematic top view of FIG. 7A, as opposed to being separatedhorizontally in a manner as illustrated in FIG. 7A. As will beappreciated from the teachings of the invention, one skilled in the artwould know how to implement an appropriate polarization-displacing unit,along with a suitable polarization-displacing unit, in a WSR apparatus,for a given application.

Moreover, the beam-modifying unit 740 may comprise an assembly ofcylindrical lenses or prisms, in optical communication with thepolarization-displacing unit 720 along with the polarization-rotatingunit 730 and the diffraction grating 101. In general, a beam-modifyingunit may be embodied by any optical structure that is capable ofmagnifying the input optical signal and de-magnifying the reflectedoptical beams according to a predetermined ratio. Such a beam-modifyingunit may be particularly useful in applications that call for a refinedspectral resolution, such as DWDM optical networking applications.

The WSR apparatus 700 of FIGS. 7A-7B is substantially similar to the WSRapparatus 100 of FIG. 1A in operation and function and hence achievesthe advantages thereof. Furthermore, the described polarizationdiversity approach renders the polarization sensitivity of thediffraction grating 101 inconsequential in the WSR apparatus 700. Thisenables the WSR apparatus 700 to minimize the insertion loss. It alsoallows the WSR apparatus 700 to enhance the spectral resolution in asimple and cost-effective construction, e.g., by making use ofhigh-dispersion holographic diffraction gratings commonly available inthe art. Another notable feature of the polarization diversity scheme isthat the first and second optical beams associated with each wavelength(corresponding to the two polarization components of each spectralchannel) effectively “exchange” their respective optical paths, uponreflection from the micromirror, i.e., the return path of the reflectedsecond optical beam is substantially similar to the forward path of thefirst optical beam, and vice versa. This has the important consequenceof substantially equalizing the overall optical paths of the twopolarization components for each spectral channel, thereby minimizingthe polarization-dependent loss (PDL) and polarization-mode dispersion(PMD). These attributes are desirable in many applications.

Those skilled in the art will appreciate that the WSR apparatus 700 ofFIGS. 7A-7B may be further modified in various ways according to theteachings of the invention. For example, the apparatus may include anarray of collimator-alignment mirrors such as described with respect tothe embodiment of FIG. 2A, 2B, or 3. FIG. 8A shows a schematic top viewof a sixth embodiment of a WSR apparatus 800A of the invention whichemploys an array of collimator-alignment mirrors 220 in a polarizationdiversity arrangement such as shown in FIGS. 7A-7B. For example, WSRapparatus 800A may be built upon the embodiments of FIGS. 2A and 7A,hence similar elements are labeled with the same numerals. In FIG. 8A,the array of collimator-alignment mirrors 220 (which may include thecollimator-alignment mirrors 220-1 through 220-N, as shown in FIG. 2A)may be disposed along the optical path between the fiber collimators 110and the polarization-displacing unit 720, such that there is aone-to-one correspondence between the collimator-alignment mirrors 220and the fiber collimators 110 providing the input and output ports. Asdescribed with respect to FIG. 2A, the collimator-alignment mirrors 220may be controlled to adjust the alignment of the input multi-wavelengthoptical signal and to further provide angular control of the collimatedbeams of the reflected spectral channels. This facilitates the couplingof the reflected spectral channels into the respective output portsaccording to desired coupling efficiencies.

In the embodiment of FIG. 8A, the collimator-alignment mirrors 220 aredisposed between the fiber collimators 110 and thepolarization-displacing unit 720, and control the angular position ofthe (un-split) multi-wavelength input optical signal as well as the(combined) reflected spectral channels. There may be applications whereit is desired to provide separate control to the first and secondpolarization components (on the forward path), as well as to thereflected first and second sets of optical beams (on the return path).FIG. 8B depicts a schematic top view of a seventh embodiment of a WSRapparatus 800B of the invention, which achieves this. WSR apparatus 800Bof FIG. 8B may be built upon the embodiment of FIG. 8A, hence similarelements are labeled with the same numerals. In this case, apolarizing-beam-splitter unit 820 may be employed instead of thepolarization-displacing unit 720 of FIG. 8A, to decompose themulti-wavelength input optical signal into first and second polarizationcomponents 822, 824 that are propagating in two orthogonal directions.The second polarization component 824 may be subsequently incident ontoand reflected by a first beam-deflecting unit 222, whereby it propagatesparallel to the first polarization component 822. The operationthereafter is substantially similar to that of FIG. 8A. On the returnpath, the reflected first set of optical beams is incident onto andreflected by the beam-deflecting unit 222, so as to enable thepolarizing-beam-splitter unit 820 to recombine the reflected first andsecond sets of optical beams by wavelength respectively into reflectedspectral channels.

In the embodiment of FIG. 8B, the polarizing-beam-splitter unit 820 maybe a single polarizing beam splitter known in the art, in opticalcommunication with the array of fiber collimators 110 via thecollimator-alignment mirrors 220. It may also comprise an array ofpolarizing beam splitters, e.g., in a one-to-one correspondence with thecollimator-alignment mirrors 220. The first beam-deflecting unit 222 maycomprise an array of first mirrors, e.g., in a one-to-one correspondencewith the collimator-alignment mirrors 220. The first mirrors 222 may beindividually adjustable, so as to control the relative alignment andthereby ensure the requisite beam parallelism between the first andsecond polarization components on the forward path, which in turnwarrants the first and second optical beams associated with eachwavelength substantially coincide on the same channel micromirror. Onthe return path, the first mirrors 222 may likewise adjust the relativealignment between the reflected first and second sets of optical beamsrespectively, thereby ensuring that the reflected first and second setsof optical beams are properly recombined into the respective spectralchannels by way of the polarizing-beam-splitter unit 820. The firstmirrors 222 may be controlled on a dynamic basis. Alternatively, thefirst mirrors 222 may be adjusted to predetermined positions to enablethe polarizing-beam-splitter unit 820 to achieve the requisite beamparallelism. The first mirrors 222 may be subsequently fixed inrespective positions over the course of operation. (In this way, thetolerances required for the polarizing-beam-splitter unit 820 may berelaxed.) It should be further appreciated that the firstbeam-deflecting unit 222 may also be a static mirror, or any otherbeam-deflecting means known in the art, configured such that thecombination of the polarizing-beam-splitter unit 820 and the firstbeam-deflecting unit 222 effectively constitutes apolarization-displacing unit as described above.

FIG. 8C depicts a schematic top view of an eighth embodiment of a WSRapparatus 800C of the invention. WSR apparatus 800C may include theelements employed in the embodiment of FIG. 8B, along with second andthird beam-deflecting units 224, 226. The second beam-deflecting unit224 may comprise an array of second mirrors that are individuallyadjustable, e.g., in a one-to-one correspondence with the first mirrorsthe first beam-deflecting unit 222 may contain. The thirdbeam-deflecting unit 226 may simply be a static mirror, or other knownbeam-deflecting device. In this way, the first and second polarizationcomponents 822, 824 (on the forward path) may be independentlycontrolled by the first and second beam-deflecting units 222, 224, whichmay also control the reflected first and second sets of optical beams(on the return path), respectively. The collimator-alignment mirrors 220may further facilitate the coupling of the (combined) reflected spectralchannels into the desired output ports.

The WSR apparatus 700 (or any one of the embodiments of FIGS. 8A-8C) ofthe invention may further incorporate a servo-control assembly, e.g., ina manner as described with respect to FIG. 4A (or 4B) above. Theservo-control assembly may dynamically manage the optical power levelsof the reflected spectral channels coupled into the output ports. Theservo-control assembly may also be configured such to minimize PDLassociated with the spectral channels.

Furthermore, a dynamically reconfigurable OADM may be built upon the WSRapparatus 700, 800A, 800B or 800C (along with an associatedservo-control assembly), e.g., in a manner similar to that describedwith respect to FIG. 5 or 6. The thus-constructed OADMs will haveimportant advantages of low insertion loss, low PDL, and enhancedspectral resolution, which would be particularly suitable for WDMoptical networking applications.

Those skilled in the art will recognize that the aforementionedembodiments are provided by way of example to illustrate the generalprinciples of the invention. Various changes, substitutions, andalternations can be made without departing from the principles and thescope of the invention as defined in the appended claims.

What is claimed is:
 1. An optical apparatus, comprising: fibercollimators providing an input port for a multi-wavelength opticalsignal and a plurality of output ports; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; and an array of channel micromirrors positioned toreflect said first and second sets of optical beams such that saidreflected first and second sets of optical beams are recombined bywavelength into reflected spectral channels by saidpolarization-rotating unit and said polarization-displacing unit;wherein said polarization-displacing unit comprises apolarization-displacing element in optical communication with said inputport and said output ports, and wherein said polarization-rotating unitcomprises a polarization-rotating element, in optical communication withsaid polarization-displacing element.
 2. The optical apparatus of claim1, wherein said polarization-displacing unit comprises apolarization-displacing element in optical communication with said inputport and said output ports.
 3. An optical apparatus, comprising: fibercollimators providing an input port for a multi-wavelength opticalsignal and a plurality of output ports; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; and an array of channel micromirrors positioned toreflect said first and second sets of optical beams such that saidreflected first and second sets of optical beams are recombined bywavelength into reflected spectral channels by saidpolarization-rotating unit and said polarization-displacing unit;wherein said polarization-displacing unit comprises a plurality ofpolarization-displacing elements in correspondence with said input portand said output ports.
 4. The optical apparatus of claim 3, wherein saidpolarization-displacing element comprises an element selected from thegroup consisting of birefringent beam displacers andpolarizing-beam-splitting elements.
 5. The optical apparatus of claim 3,wherein said polarization-rotating unit comprises a plurality ofpolarization-rotating elements in correspondence with saidpolarization-displacing elements.
 6. The optical apparatus of claim 5,wherein each polarization-rotating element comprises an element selectedfrom the group consisting of half-wave plates, Faraday rotators, andliquid crystal rotators.
 7. An optical apparatus, comprising: fibercollimators providing an input port for a multi-wavelength opticalsignal and a plurality of output ports; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; an array of channel micromirrors positioned to reflectsaid first and second sets of optical beams such that said reflectedfirst mid second sets of optical beams are recombined by wavelength intoreflected spectral channels by said polarization-rotating unit and saidpolarization-displacing unit; and a beam-modifying unit for providinganamorphic beam magnification of said first and second polarizationcomponents and anamorphic beam demagnification of said reflected firstand second sets of optical beams.
 8. The optical apparatus of claim 7,wherein beam-modifying unit comprises one or more cylindrical lenses. 9.The optical apparatus of claim 7, wherein beam-modifying unit comprisesone or more prisms.
 10. An optical apparatus, comprising: fibercollimators providing an input port for a multi-wavelength opticalsignal and a plurality of output ports; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; an array of channel micromirrors positioned to reflectsaid first and second sets of optical beams such that said reflectedfirst and second sets of optical beams are recombined by wavelength intoreflected spectral channels by said polarization-rotating unit and saidpolarization-displacing unit; and an array of collimator-alignmentmirrors in optical communication with said fiber collimators and saidpolarization-displacing unit for adjusting an alignment of saidmulti-wavelength optical signal from said input port and for directingsaid reflected spectral channels into said output ports.
 11. The opticalapparatus of claim 10, wherein each collimator-alignment mirror isrotatable about at least one axis.
 12. An optical apparatus, comprising:fiber collimators providing an input pod for a multi-wavelength opticalsignal and a plurality of output pods; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; and an array of channel micromirrors positioned toreflect said first and second sets of optical beams such that saidreflected first and second sets of optical beams are recombined bywavelength into reflected spectral channels by saidpolarization-rotating unit and said polarization-displacing unit;wherein said polarization-displacing unit comprises a polarizing beamsplitter and a first beam-deflecting unit.
 13. The optical apparatus ofclaim 12, wherein said first beam-deflecting unit comprises an array offirst mirrors that are individually adjustable to control positions ofsaid second polarization component and said reflected first set ofoptical beams.
 14. The optical apparatus of claim 13, further comprisinga second beam-deflecting unit, in optical communication with said firstpolarization component and said reflected second set of optical beams,said second beam-deflecting unit comprising an array of second mirrorsthat are individually adjustable.
 15. An optical apparatus, comprising:fiber collimators providing an input port for a multi-wavelength opticalsignal and a plurality of output ports; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; an array of channel micromirrors positioned to reflectsaid first and second sets of optical beams such that said reflectedfirst and second sets of optical beams are recombined by wavelength intoreflected spectral channels by said polarization-rotating unit and saidpolarization-displacing unit; and a servo-control assembly, including aspectral monitor for monitoring optical power lever of said reflectedspectral channels and a processing unit responsive to said optical powerlevels for controlling said channel micromirrors.
 16. The opticalapparatus of claim 15, wherein said servo-control assembly controls saidchannel micromirrors to maintain said optical power levels at apredetermined value.
 17. An optical apparatus, comprising: fibercollimators providing an input port for a multi-wavelength opticalsignal and a plurality of output ports; a polarization-displacing unitthat decomposes said multi-wavelength optical signal into first andsecond polarization components; a polarization-rotating unit thatrotates a polarization of the second polarization component to besubstantially parallel to a polarization of the first polarizationcomponent; a wavelength-separator that separates said first and secondpolarization components by wavelength into first and second sets ofoptical beams; and an array of channel micromirrors positioned toreflect said first and second sets of optical beams such that saidreflected first and second sets of optical beams are recombined bywavelength into reflected spectral channels by saidpolarization-rotating unit arid said polarization-displacing unit;wherein each channel micromirror is pivotable about two axes; andwherein said fiber collimators are arranged in a two-dimensional array.18. An optical apparatus, comprising: fiber collimators providing aninput port for a multi-wavelength optical signal arid a plurality ofoutput ports; a polarization-displacing unit that decomposes saidmulti-wavelength optical signal into first and second polarizationcomponents; a polarization-rotating unit that rotates a polarization ofthe second polarization component to be substantially parallel to apolarization of the first polarization component; a wavelength-separatorthat separates said first and second polarization components bywavelength into first and second sets of optical beams; and an array ofchannel micromirrors positioned to reflect said first and second sets ofoptical beams such that said reflected first and second sets of opticalbeams are recombined by wavelength into reflected spectral channels bysaid polarization-rotating unit and said polarization-displacing unit;wherein said array of channel micromirrors reflects said first andsecond sets of optical beams so as to couple said beams into selectedoutput ports.
 19. An optical apparatus, comprising: fiber collimatorsproviding an input pod for a multi-wavelength optical signal and aplurality of output ports; a polarization-displacing unit thatdecomposes said multi-wavelength optical signal into first and secondpolarization components; a polarization-rotating unit that rotates apolarization of the second polarization component to be substantiallyparallel to a polarization of the first polarization component; awavelength-separator that separates said first and second polarizationcomponents by wavelength into first and second sets optical beams; andan array of channel micromirrors positioned to reflect said first andsecond sets of optical beams such that said reflected first and secondsets of optical beams are recombined by wavelength into reflectedspectral channels by said polarization-rotating unit and saidpolarization-displacing unit; wherein said fiber collimators arearranged in a one-dimensional array.
 20. An optical apparatus,comprising: fiber collimators providing an input port for amulti-wavelength optical signal and a plurality of output ports; apolarization-displacing unit that decomposes said multi-wavelengthoptical signal into first and second polarization components; apolarization-rotating unit that rotates a polarization of the secondpolarization component to be substantially parallel to a polarization ofthe first polarization component; a wavelength-separator that separatessaid first and second polarization components by wavelength into firstand second sets of optical beams; an array of channel micromirrorspositioned to reflect said first and second sets of optical beams suchthat said reflected first and second sets of optical beams arerecombined by wavelength into reflected spectral channels by saidpolarization-rotating unit and said polarization-displacing unit; and abeam-focuser for focusing said first and second sets of optical beamsonto said channel micromirrors.
 21. A method of dynamic routing of amulti-wavelength optical signal in a polarization diversity arrangementcomprising: decomposing said multi-wavelength optical signal into firstand second polarization components; providing an anamorphic beammagnification to said first and second polarization components,respectively; rotating a polarization of said second polarizationcomponent to be substantially parallel to a polarization of the firstpolarization component; separating said first and second polarizationcomponents by wavelength respectively into first and second sets ofoptical beams; focusing said first and second sets of optical beams ontoan array of micromirrors; dynamically controlling said micromirrors toreflect said first and second sets of optical beams into selected outputports; rotating a polarization of said reflected First, set of opticalbeams of approximately 90-degrees; and recombining said reflected firstand second sets of optical beams by wavelength into reflected spectralchannels.
 22. The method of claim 21 further comprising the step ofmonitoring said optical power levels at a predetermined value.
 23. Amethod of dynamic routing of a multi-wavelength optical signal in apolarization diversity arrangement, comprising: decomposing saidmulti-wavelength optical signal into first and second polarizationcomponents; rotating a polarization of said second polarizationcomponent to be substantially parallel to a polarization of the firstpolarization component; separating said first, and second polarizationcomponents by wavelength respectively into first and second sets ofoptical beams; focusing said first and second sets of optical beams ontoan array of micromirrors; dynamically controlling said micromirrors toreflect said first and second sets of optical beams into selected outputports; rotating a polarization of said reflected first set of opticalbeams by approximately 90-degrees; recombining said reflected first andsecond sets of optical beams by wavelength into reflected spectralchannels monitoring optical power levels of said reflected spectralchannels coupled into said output pods; and providing Feedback controlof said micromirrors.
 24. The method of claim 23 further comprising thestep of maintaining said optical power levels at a predetermined value.25. A method of dynamic routing of a multi-wavelength optical signal ina polarization diversity arrangement, comprising: adjusting an alignmentof said multi-wavelength optical signal; decomposing saidmulti-wavelength optical signal into first and second polarizationcomponents; rotating a polarization of said second polarizationcomponent It) be substantially parallel to a polarization of the firstpolarization component; separating said first and second polarizationcomponents by wavelength respectively into U) first and second sets ofoptical beams; focusing said first and second sets of optical beams ontoan array of micromirrors dynamically controlling said micromirrors toreflect said first and second sets of optical beams into selected outputports; rotating a polarization of said reflected first set of opticalbeams by approximately 90-degrees; and recombining said reflected firstand second sets of optical beams by wavelength into reflected spectralchannels.
 26. The method of claim 25 further comprising the step ofcoupling of said reflected spectral channels into selected output ports.27. A method of dynamic routing of a multi-wavelength optical signal ina polarization diversity arrangement, comprising: decomposing saidmulti-wavelength optical signal into first and second polarizationcomponents; adjusting a relative alignment between said first and secondpolarization components; rotating a polarization of said secondpolarization component to be substantially parallel to a polarization ofthe first polarization component; separating said first and secondpolarization components by wavelength respectively into first and secondsets of optical beams; focusing said first and second sets of opticalbeams onto an array of micromirrors; dynamically controlling saidmicromirrors to reflect said first awl second sets of optical beams intoselected output pods; rotating a polarization of said reflected firstset of optical beams by approximately 90-degrees; and recombining saidreflected first and second sets of optical beams by wavelength intoreflected spectral channels.
 28. The method of claim 27 furthercomprising the step of adjusting a relative alignment between saidreflected first and second sets of optical beams.